In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) Resonator. The BAW resonator has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The BAW resonator includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
BAW resonators are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the BAW resonators have thicknesses on the order of microns and length and width dimensions of hundreds of microns, BAW resonators beneficially provide a comparatively compact alternative to known resonators.
Desirably, the bulk acoustic resonator excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., z-direction) of the piezoelectric layer.
Unfortunately, besides the desired TE modes there are lateral modes, known as Rayleigh-Lamb modes, generated in the acoustic stack as well. The Rayleigh-Lamb modes are mechanical waves having k-vectors that are perpendicular to the direction of TE modes, the desired modes of operation. These lateral modes travel in the areal dimensions of the piezoelectric material. Among other adverse effects, lateral modes deleteriously impact the quality (Q) factor of a BAW resonator device. In particular, the energy of Rayleigh-Lamb modes is lost at the inactive region and at the interfaces of the BAW resonator device. As will be appreciated, this loss of energy to spurious modes is a loss in energy of desired longitudinal modes, and ultimately a degradation of the Q-factor.
BAW resonators comprise an active area, and connections to and from the active area can increase losses, and thereby degrade the Q factor. For example, in transition regions between the active area and the connections, defects may form in the piezoelectric layer during fabrication as a result of the termination of the lower electrode of the BAW resonator structure. These defects can result in acoustic loss, and as a result a reduction in the Q factor.
What is needed, therefore, are an acoustic resonator structure electrical filter that overcomes at least the known shortcomings described above.